Snowmobile Fanatics banner
1 - 5 of 5 Posts

·
Registered
Joined
·
2,450 Posts
Discussion Starter · #1 ·
here is a type up to help you understand the principles and such of of clutching details.

compliments of mxzx-revzone. (I did the editing though).

Clutching definitions and principles


Spring "Rate" & "Forces":

When it comes to springs and their forces, I always hear or read the word "rate" when the word "Force" should be used. Here people describe a quantity of a spring and are not referring right to the element of the spring.
Rate is the fixed ratio between two things. In the case of a spring, rate is the quantity pounds (Ibs) measured per inch of change. If you compress the spring, it takes "x" amount of pounds of force (per inch) to change the height of the spring. The starting point is the installed height (the height of the spring with the Primary clutch cover face-plate bolted on) and the finish point is the fully compressed height (the height of the spring when the engine is at max RPM or WOT).
-Start force (primary & secondary spring) the value of the spring in lbs. force when it is installed.
-Final force (primary & secondary spring) the amount of lbs. force when the clutch is at full shift.

Force, in this case, is the strength to hold something in position, for example a 170/300 primary spring; the spring needs 170 lbs of force to keep it at installed height and must add enough force of up to 300 lbs to change its position to full compressed height..

**The spring does not have 170 lbs "rate" at the beginning of the shift**

-The spring at installation is compressed 1.25" and requires 170 lbs of force to keep it that position.
-The spring at full shift is compressed to 2.5" and requires 300 lbs of force to keep it that position.

To figure out the rate of the spring you do a simple calculation:
a] 300 lbs - 170 lbs = 130 lbs
b] 2.5" - 1.25" = 1.25" of distance travel between engagement and full shift.
130 lbs ÷ 1.25" distance = 104 lbs per inch of rate.

From installation height of 1.25" you want to compress the spring just 1" more (going faster/accelerating), you would have to add another 104 lbs of force on top of the initial 170 lbs to make the spring compressed to 2.25"(not full shift/not WOT). To keep the spring compressed at 2.25" that would mean you did 170 Ibs + 104 Ibs = 274 lbs to keep it in that position.


"force" and "rate" example(s):

Q1] if you change to a softer spring (one with less finish rate in the primary). Softer primary spring has faster up shift.
A1] using as an example; with a 170/300 spring, the idea given here is to lose the 300 and go with something lower. For lack of better numbers let’s say "260". This is a change to a 170-260 from a 170-300.
*******Changing to a softer spring-one with less finish "force" in the primary, that softer primary spring has a faster up shift.


Backshift:
-Specifying what you observe while the sled is in motion; usually under full throttle...
You read scenarios like: weak/strong...
Scenarios:
1) weak...come around the corner, pressed the throttle and the engine would hit 500 rpms low and slowly accelerate.
strong...come around the corner, punch the throttle and the engine is right there, whammo at 8000 rpm.
2) weak...accelerates hard to top end, but if the snow load increases (gets deeper) the engine rpms starts to drop (commonly referred to as a ‘bog’).
strong...accelerates hard to top end, except when the snow load increases (gets deeper), the engine rpms stays the same (maintains the same WOT rpm)
3) weak...good acceleration but if I burp the throttle the engine comes back in at low rpms.
strong...good acceleration and when I burp the throttle, bam the engine hits the right rpms.
4) weak...while climbing a hill the rpms are 1000 rpms lower than they should be
strong...while climbing my rpms stay rock steady at the rated rpm for WOT
5) weak...when I land after a drift(jump) the rpms come in low and won't recover until I slow down, and re-accelerate
strong...when I land after a drift(jump), the rpms come in low but recover while I keep on the throttle.
IF There are opinions of backshift using adjectives like "strong" or "weak", example....."strong backshift" or "weak backshift"
THEN could you define - Backshift: is the ability for the engine to accelerate to rated rpms, as the load changes.




Aggressive
Many times I see confusion regarding the word "Aggressive" when it comes to clutching.
People relate this word when they talk of how the clutches shift.
Regarding the primary- Aggressive: The clutch exerts more forceful energy using less engine rpm.
The clutch uses more of the meat of the torque curve from the engine, shifing harder with lower rpms.
When you quickly cycle the throttle(burp it), the engine is not revvy, but the sled responds with greater movement from your thumb action.
You alter the feeling of the sled with greater action in the bottom end of the midrange when pressing the throttle.


Temperature
Regardless of temperature you measure-whether it’s hot, cold, warm, burning hot, freezing cold- The lowest temperature you'll measure is when having clutch calibration allow the engine to run at "rated rpms" on a sustained wide open throttle pull.


Engagement rpms
(Primary spring) - If you want to raise engagement rpms, THEN increase the primary spring start force. Example: if engagement with a 170/XXX spring is 4000rpm, putting in a 190/XXX spring will bring engine rpms to 5000 rpm (**NOT actual specs/output numbers, just an example in theory**).


Rpms near full shift-springs
(Primary & secondary spring) If rpms are low or diminish(drop) as shift increase, THEN increase final force of the primary or secondary spring.
Example, If the rpms are 250rpm lower than they should be while using a (XXX / 300), THEN change the spring to a (XXX / 320).


Rpms near full shift-helix
(Helix final angle) If rpms are low or diminish (drop) as shift increase, THEN reduce the helix angle "at that point" on the helix.
Example, IF rpms start to reduce from rated while using a (XX / 56) degree angle, THEN reduce that angle to a lower one, like a (XX / 52).


Upshift rpms-springs
(Primary & secondary spring) If the rpms are low as sled speed increases, THEN increase start force of the primary or secondary spring.
Example, If upshift rpms are 400rpms low and slowly increase to rated rpms when using a (130 / XXX), THEN increase the start force to (160 / XXX).


Upshift rpms-helix
(Helix start angle) If the rpms are low as sled speed increases, THEN reduce the helix start angle.
Example, If upshift rpms are 200rpm low with a (44 / XX) start angle, THEN reduce the start angle to 40 degrees (40 / XX)
 

·
Registered
Joined
·
2,450 Posts
Discussion Starter · #2 ·
CRAP. i forgot about the clutching sub-forum. Can a mod move this there?
 

·
Super Moderator
Joined
·
11,680 Posts
Moved to Clutching.
 

·
Registered
Joined
·
2,731 Posts
tughillboy said:
Thanks for posting this. Really helpful as i'm thinking about messing with my clutches over the summer. [thumb]
Do it! I also highly recommend Olav Aaen's clutch tuning handbook. read it twice, highlite the important stuff.
I clutched mine this last month, already VERY impressed with initial results. But I won't be sure if I have to adjust some things until I ride it on the trails this winter (have to test max RPM, backshifting, midrange, etc.)
 
1 - 5 of 5 Posts
Top